1. Field of The Invention
This invention relates to an image processing system for performing a processing such as a tone correction on image data optically read from a photograph or the like and thereafter outputting processed image data.
2. Description of The Related Art
In recent years, with advance of image processing techniques, an image processing system, which can read image data of a picture or the like and perform various processings on the image data and also can edit the image data by, for example, "pasting" or placing an image represented by the image data on a document, has come to require a variety of functions and an improvement in operability.
A conventional image processing system has a function of a tone correction, by which a tone of read image data is converted into another tone arbitrarily indicated by an operator. In such a case, a desired tone-correction rate or degree can be set by first displaying a curve graph (hereunder sometimes referred to as a tone curve or as a tone correction curve), in which tone levels (namely, densities) of input image data are plotted in abscissa (namely, in horizontal axis) and tone levels of output image data obtained by performing a tone correction on the tone of the input image data are plotted in ordinate (namely, in vertical axis), as illustrated in FIG. 39 and next modifying the tone curve by using an input means such as a mouse. In case of FIG. 39, a tone curve 1 corresponds to a case where a tone level of output image data is equal to a tone level of corresponding input image data (namely, output image data is obtained from input image data without any conversion or modification). Further, as shown in FIG. 39, a tone curve 2 is entirely lower than the tone curve 1. Thus, in case of employing the tone curve 2, an output image represented by the output image data is lighter than an input image represented by the input image data. In contrast, a tone curve 3 is entirely higher than the tone curve 1. Thus, in case of employing the tone curve 3, an output image represented by the output image data is darker than an input image represented by the input image data. Further, a portion of an output image, which is represented by tone levels of a steep portion of a tone curve, is enhanced in comparison with a corresponding input image. Conversely, another portion of the output image, which is represented by tone levels of a gentle portion of the tone curve, is blurred or scumbled in comparison with the corresponding input image.
To set such a tone curve, information on densities at arbitrary positions (namely, of arbitrary pixels) of an original image (or an input image) and the distribution of densities of pixels of the original image is necessary. Thus, in case of a conventional image processing system, multi-level density data (hereunder sometimes referred to as multi-level image data) obtained as a result of a tone correction performed on image data read from the original image is not only converted into binary data (or halftone-dot data) to be displayed on a screen of a cathode-ray-tube (CRT) of a display device thereof, such multi-level data itself is also stored in a random access memory (RAM) thereof.
Hereinafter, a conventional image processing system will be described.
FIG. 40 is a schematic block diagram for illustrating the configuration of the conventional image processing system. Reference numeral 21 designates a central processing unit (CPU) for controlling the system and processing data; 22 a RAM for storing 8-bit 256-level image data, which represents the density of each pixel with 256 levels, as the above described multi-level density data; 23 what is called a page memory for storing binary data converted from the multi-level image data stored in the RAM 22 under the control of the CPU 21; 24 an image input unit for optically reading a draft such as a photograph, for performing a tone correction on the read image data and for transferring the corrected data to the RAM 22; 25 a CRT display unit; and 26 a printer.
FIG. 41 is a schematic block diagram for illustrating the configuration of the image input unit 24 of the image processing system of FIG. 40. Reference numeral 27 denotes a CCD image sensor (hereunder referred to as a CCD) for reading image data from a draft picture optically and outputting an analog electric signal representing the read image data; 28 an analog-to-digital (A/D) converter for converting an analog voltage signal output from the CCD 27 to a digital signal; 29 a shading correction circuit for performing a shading correction on the digital signal; 30 a tone correction circuit for performing a tone correction an the signal according to a tone-correction rate preliminarily set by the CPU 21.
Hereinafter, an operation of the conventional image processing system having the above described configuration will be described by referring to FIGS. 40 and 41.
First, image data optically read by the CCD 27 and converted into an analog electric signal is further converted by the A/D converter 28 into an 8-bit (256-level) digital electric signal. Then, the digital electric signal undergoes a shading correction in the shading correction circuit 29. Subsequently, the tone correction circuit 30 performs a tone correction on the data represented by the signal corrected in the circuit 29 at the predetermined tone-correction rate. Next, the corrected data is written to the RAM 22. Then, the 8-bit image data stored in the RAM 22 is converted by the CPU 21 into binary image data (or halftone-dot image data) which is then displayed on the CRT display unit 25 and is also output to the printer 26.
The tone-correction rate can be determined as equal to a rate desired by an operator by setting a tone curve used for a tone correction. In the conventional image processing system, the read multi-level image data (namely, 8-bit (256-level) image data) respectively representing density-levels of all pixels of a draft picture (namely, an input image) are stored in the RAM. For example, the stored image data are accessed for inquiring into the densities of desired points of the input image displayed on the CRT display unit and into the distribution of densities (namely, tone-levels) represented by the image data. Namely, the stored image data are referred to for setting a tone curve for a tone correction.
The conventional image processing system, however, has a drawback in that a large number of bits are required for storing density data corresponding to each pixel and thus a storage device having a huge storage capacity is necessary. The present invention is created to eliminate such a drawback of the conventional system.